how safe to dig near concrete slab

I assume the garage floor is insulated, not that it matters. I doubt the heat from a compost pile will have much affect on the garage floor or foundation. My concern would be from water in the pit.

The radiant elements are in the slab and should be away from the edge. Your digging outside the garage should have no connection with the heating elements.
Ron

Your garage floor will very likely be supported underneath with a foundation. This will rest on a footing that is 4' down.
Your excavation is highly unlikely to have any effect on the garage floor.
As you dig your pit, you will likely see the foundation wall.

thanks for your replies

The pit is dug. Only problem is that it is not big enough to get through our long winter for a collection of compost. I might need to dig another! roberta.

Seems like a bad idea to have a compost pile/pit in close proximity to the house. Vermin and rot are not things to attract to the house.

the garage is detached, the pit on the far side of the garage from the house. There is no meat in my compost. The only thing to fear is bears.....yes bears. They eat anything. and they can get into anything they want. We live in the woods in northern Maine. We have a large dog. So far...no termites this far north, just carpenter ants ...we already have those. mostly compost smells and has various flies. this plan has a vent pipe with a screen for fly contro.l the worms are a helpful hopeful guests.